(1) Field of the Invention
This invention pertains to electrolytic capacitors, particularly to liquid electrolyte, tantalum and silver case capacitors, having a codeposited noble metal/base metal cathode element.
(2) Description of Related Art
The counterelectrode in an electrolytic capacitor must present a very low impedance to an AC signal if the capacitance of the anode element is to be determining factor in the capacitance of the finished capacitor. This is often accomplished by the use of a porous tantalum sintered body with high specific capacitance in tantalum case capacitors or by the deposition of a spongy layer of a platinum group metal in silver case capacitors.
The ideal counterelectrode in an electrolytic capacitor would make no contribution to the overall capacitance of the device. Presently, there are three primary approaches to minimizing the impedance attributable to the counterelectrode. Use of an anodized tantalum counterelectrode results in a non-polar, but usually highly asymmetrical, device. Capacitors produced with this technology are limited in capacitance and are relatively expensive to manufacture.
Alternatively, chemical or electrochemical deposition of a noble metal on the inner surface of the capacitor case has been applied to silver case capacitors. This approach is also severely limited in attainable capacitance and is often unpredictable. Finally, addition of an electroactive species to the capacitor to induce a low impedance electrochemical reaction to occur at the counterelectrode-electrolyte interface is taught by Booe in U.S. Pat. No. 2,834,926.
U.S. Pat. No. 2,834,926 (Booe) discloses a tantalum electrolytic capacitor with an iron chloride depolarizer. A large amount of the depolarizer is dissolved in the electrolyte to minimize losses in the capacitor.
U.S. Pat. No. 3,082,360 (Robinson et al) discloses an electrolytic capacitor with a composite cathode comprising a noble metal container with a spongy layer of noble metal particles deposited on the inner surface of the container.
U.S. Pat. No. 3,243,316 (O'Nan et al) discloses tantalum electrolytic capacitors in which the cathode has a thin film of colloidal non-metallic conductive material deposited thereon to effect cathode depolarization.
U.S. Pat. No. 3,349,295 (Sparkes) discloses a tantalum capacitor having a porous cathode treated with platinum black for depolarization.
U.S. Pat. No. 3,523,220 (Harding) discloses treating the interior of a silver case by electrolyzing a plating solution containing silver ions and at least one member of the platinum family ions to codeposit silver and the platinum family metal on the case.
U.S. Pat. No. 3,810,770 (Bianchi et al) discloses titanium or tantalum base electrodes with applied titanium or tantalum oxide faces activated with noble metals. Mixtures of valve metals, platinum family metals, and optional tin and aluminum doping metals are thermally deposited on anodes. The process is said to be particularly useful in flowing mercury cathode cells. Only thermal deposition is exemplified, always with a valve metal ion present in solution; utility for capacitors is not indicated.
U.S. Pat. No. 4,020,401 (Cannon et al) discloses an electrolytic capacitor having a silver plated nickel case. A porous layer of a platinum group metal is bonded to the silver plating.
U.S. Pat. No. 4,159,509 (Walters) discloses a capacitor including a cathode with a porous layer of a platinum group metal and a cathode depolarizer dissolved in the electrolyte including copper ions reducible to copper metal.
U.S. Pat. No. 4,780,797 (Libby et al) discloses a highly efficient counterelectrode for tantalum capacitors having an alloyed layer of tantalum with a platinum family metal formed on the inner tantalum surface of the case.
None of the aforementioned techniques have enabled the production of reliable capacitors with a volumetric efficiency greater than about 2500 microfarad-volt per cubic centimeter. This limitation has heretofore precluded significant miniaturization of capacitor devices.